1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory.
2. Description of the Related Art
The size of a memory cell of a nonvolatile semiconductor memory, for instance, a flash memory, is undergoing microfabrication in accordance with an increase of a memory capacity, while it becomes necessary for the memory cell not to lower a coupling ratio.
As for a technique to increase the coupling ratio of the memory cell, in an aspect of the device structure, for instance, there is a structure (hereinafter referred to as three-dimensional cell structure) in which a sidewall of a channel length direction (a direction orthogonally crossing the extending direction of a word line as a control gate electrode) of a floating gate electrode is covered by the control gate electrode.
However, in the three-dimensional cell structure, problems of interference between adjacent two cells, property of embedding an insulation film in a narrow space between adjacent two cells, and the like become conspicuous due to the microfabrication of the memory cell. Thus, when considering such problems, a structure (hereinafter, referred to as two-dimensional cell structure) in which a side surface in a channel length direction of the floating gate electrode is not covered by a control gate electrode, is more favorable than the three-dimensional cell structure.
In the two-dimensional cell structure, in order to increase the coupling ratio, in the aspect of materials, for instance, high dielectric constant materials, called high-k materials, having higher dielectric constant than SiO2/SiN/SiO2 (hereinafter referred to as an ONO film) is used as an inter-electrode insulator between the floating gate electrode and the control gate electrode (for instance, refer to JP-A 2006-203200 (KOKAI).
Here, in the two-dimensional structure, it should be noted that a high electric field is applied to an inter-electrode insulator. In other words, the inter-electrode insulator necessitates characteristics in which the inter-electrode insulator has high dielectric constant, and has less leakage current at a high electric field region.
This can also be applied to a memory cell, for instance, a memory cell with a MONOS structure, comprised of an insulating film whose charge trap layer has a charge trap function. That is, a blocking insulator between the charge trap layer and the control gate electrode necessitates characteristics in which the blocking insulator has high dielectric constant, and has less leakage current at a high electric field region.
However, in the progress of microfabrication of the memory cell, until now, sufficient examination has not been performed with respect to materials having such characteristics.